The present invention relates to processes for preparing bead polymers with an average particle size in the range from 1 to 40 xcexcm, in which a polymerizable mix is dispersed in an aqueous phase and polymerized. The present invention also relates to moulding compositions, mouldings and PAMA plastisols which include the bead polymers prepared according to the invention.
In various applications there is a need for bead polymers whose particle diameters are of the order of size from 1 to 40 xcexcm with relatively narrow particle size distribution. Such beads are useful additives for polymethyl methacrylate (PMMA) moulding compositions and for PAMA plastisols.
A particular field of application is light-scattering moulding compositions, where standard moulding compositions are blended with what are known as scattering beads, which are crosslinked and have a refractive index different from that of the matrix. Scattering particles based on PMMA and having a particle size well above 40 xcexcm are currently used in these moulding compositions. The advantage of these scattering particles is the high level of forward scattering by the mouldings after incorporation of the scattering particles into the moulding compositions. This achieves a substantially higher luminous efficiency than that given by traditional opacifiers, e.g. BaSO4 or TiO2, at high scattering levels, since there is less loss due to backward scattering. This preferred forward scattering can be determined by measuring the transmittance combined with the halved-energy angle of mouldings which include the scattering beads.
The smaller the size of the scattering beads, the greater their scattering effect. The amount of beads used can be reduced if the beads are smaller. This reduction in the amount of beads saves costs and conserves resources. Moulding compositions which contain the smaller bead polymers have excellent mechanical properties, since the reduced amount of scattering beads has less effect on these properties. There is a problem, however, in that the perceived yellowness of the resultant moulding compositions increases markedly if scattering beads having a diameter less than 5 xcexcm are used.
The beads described above can also be used for matt moulding compositions by blending the crosslinked beads with standard moulding compositions. When these moulding compositions are extruded the resultant articles have a velvety matt surface. A particular advantage of matt surfaces is that, unlike glossy surfaces, they make fingerprints and dirt difficult to see. A decisive factor in this application is that the particle size distribution of the beads is narrow. The smaller the particles themselves, the finer is the impression given by the surface.
A specific experiment in which a cast PMMA sheet that includes light-scattering particles is illuminated laterally reveals that the ideal particles have a size in the range from 5 to 20 xcexcm, with very narrow size distribution.
The abovementioned beads may also be incorporated into polyalkyl (meth)acrylate (PAMA) plastisols in which the polymer particles having no or very little crosslinking, generally spray-dried emulsion polymers and/or beads, are mixed into a plasticizer. This gives what are known as plastisol pastes. These pastes are then applied and heated, whereupon the beads are solvated by the plasticizer so that cooling gives what is known as a plastisol film. Among the parameters of decisive importance for the formulation of the plastisol is paste stability, since the process known as gelling occurs even prior to heating of the paste. To increase paste stability, PMMA beads with particle diameter of about 40 xcexcm are currently incorporated into the pastes together with the conventional, spray-dried emulsion polymers. A disadvantage of these beads is that they do not gel completely during the short heating phase, and this results in inhomogeneous plastisol films. The only ways of reducing the problem currently are to lengthen the heating phase or use a paste with low bead concentration.
Reducing the particle size to values in the range from 1 to 40 xcexcm, together with a narrow bead distribution, enables a larger proportion of beads to be incorporated into the paste, thus improving paste stability.
Polymer particles whose size is of the order of from 1-10 xcexcm can readily be prepared by precipitation polymerization, using large amounts of organic solvents. However, the handling of the solvents used creates safety and disposal problems. Work-up processes are also difficult. Beads obtained in this way are therefore expensive and cost reasons prevent their use in the application sectors described above.
Polymer beads can be obtained at lower cost by conventional suspension polymerization. However, the size of the resultant particles is generally greater than 40 xcexcm, with broad distribution.
EP-A-0 443 609, for example, discloses a suspension process for preparing bead polymers by using a mixing cell with high shear energy to combine two phases introduced separately (monomers and continuous phase) and then polymerizing the monomers in a conventional reaction vessel. Various auxiliaries are mentioned for stabilizing the dispersion. These include inorganic substances, such as calcium phosphate, and organic compounds, such as cellulose derivatives or polyvinyl alcohol. EP-A-0 443 609 does not describe the use of aluminum compounds.
The examples in EP-A-0 443 609 state that particle sizes in the range from 5 to 10 xcexcm are obtained. However, the monomer mixture was composed of 80% by weight of styrene. If these experiments are repeated using mixtures which include more than 50% by weight of acrylic monomers the particles become much larger. Although the use of known organic stabilizers gives small particles, if these particles are processed into moulding compositions marked yellowing occurs, causing increased reflection, i.e. reduced luminous efficiency.
One object of the present invention is to avoid the problems in conventional processes.
Another object of the present invention is to provide a process for preparing bead polymers, which results in particles with an average size in the range from 1 to 40 xcexcm, and which on incorporation into moulding compositions give little yellowing.
Another object of the present invention is to provide a process for preparing bead polymers, which results in particles with an average size in the range from 1 to 40 xcexcm, and which does not require large amounts of organic solvents that would require removal after the polymerization.
Another object of the invention is to provide a process which can be carried out using commercially available systems.
Another object on which the invention is to provide a process which can be implemented without any major safety risk.
Another object on which the invention is to provide a process which can be carried out without release or handling of pollutants.
Another object of the present invention is to provide a process which can be carried out at low cost.
Another object of the present invention is to provide a process for preparing bead polymers, which results in particles with an average size in the range from 1 to 40 xcexcm, and in which the suspension obtained can be worked up by filtration.
Another object of the present invention is to provide a process for preparing bead polymers, which results in particles with an average size in the range from 1 to 40 xcexcm, and in which the suspension obtained can be worked up by filtration under superatmospheric pressure.
Another object of the present invention is to provide a process for preparing bead polymers, which results in particles with an average size in the range from 1 to 40 xcexcm, and in which the suspension obtained can be worked up by filtration under sub-atmospheric pressure.
Another object of the present invention is to provide moulding compositions which have excellent properties and can be produced at low cost.
Another object of the present invention is to provide PAMA plastisols and mouldings which have excellent properties and can be produced at low cost.
These and other objects of the invention have now been achieved by the present invention, the first embodiment of which provides a process for preparing bead polymers having an average particle size of 1 to 40 xcexcm, which includes:
contacting:
at least one polymerizable mix which includes at least 50% by weight of at least one (meth)acrylate monomer,
at least one aluminum compound, and
an aqueous phase,
to prepare a mixture;
dispersing the mixture at a shear rate xe2x89xa7103 sxe2x88x921 to form a dispersion, wherein the dispersion is stabilized by the aluminum compound; and
polymerizing to produce bead polymers having an average particle size of 1 to 40 xcexcm.
Another embodiment of the present invention provides the mixture prepared by the abovementioned process.
Another embodiment of the present invention provides the dispersion prepared by the abovementioned process.
Another embodiment of the present invention provides the bead polymers prepared by the abovementioned process.
Another embodiment of the present invention provides a PAMA plastisol, which includes the bead polymers prepared by the abovementioned process.
Another embodiment of the present invention provides a dental composition, which includes the bead polymers prepared by the abovementioned process.
Another embodiment of the present invention provides a porous mould, which includes at least one plastic and the bead polymers prepared by the abovementioned process.
Another embodiment of the present invention provides a moulding composition, which includes the bead polymers prepared by the abovementioned process.
Another embodiment of the present invention provides a moulding having at least one matt surface, which includes the bead polymers prepared by the abovementioned process.
Another embodiment of the present invention provides a composition, which includes the bead polymers prepared by the abovementioned process in contact with at least one matrix polymer.
Another embodiment of the present invention provides a process for preparing bead polymers having an average particle size of 1 to 40 xcexcm, which includes:
contacting:
at least one polymerizable mix which includes at least 50% by weight of at least one (meth)acrylate monomer,
at least one means for stabilizing a dispersion, and
an aqueous phase,
to prepare a mixture;
dispersing the mixture at a shear rate xe2x89xa7103 sxe2x88x921 to form the dispersion, wherein the dispersion is stabilized by the means for stabilizing; and
polymerizing to produce bead polymers having an average particle size of 1 to 40 xcexcm.